1. Field of the Invention
The present invention relates generally to a massive Multiple-Input Multiple-Output (MIMO) system operating in a Frequency Division Duplex (FDD) mode and, more particularly, to a method and apparatus for efficiently transmitting/receiving a Channel State Information Reference Signal (CSI-RS) with a limited amount of downlink resource in the massive MIMO wireless communication system.
2. Description of the Related Art
In the massive MIMO technique, a base station equipped with a specified number, such as, for example, a few hundred, transmit antennas transmits data to a plurality of mobile stations with a multi-user MIMO scheme. Since the transmit beamforming gain is in proportion to the number of transmit antennas and it is possible to reduce the transmit power of each antenna with the high beamforming gain expected with the massive MIMO system, the massive MIMO has emerged as an important “green” communication technology.
In order to form the transmit beams of the massive MIMO system, the transmitter must receive downlink Channel State Information (CSI) for each downlink. In a Time Division Duplexing (TDD) system, a downlink channel estimation technique has been proposed that is based on a Sounding Reference Signal (SRS). However, in this technique, an uplink SRS's vulnerability to inter-cell interference degrades the channel estimation performance, resulting in reduction of system throughput.
In the traditional FDD Long Term Evolution-Advanced (LTE-A) system, the CSI-RS symbols are mapped to the Resource Elements (REs) on different carriers in a frequency domain, and at different Orthogonal Frequency Division Multiplexing (OFDM) symbols in a time domain, or are multiplied by orthogonal codes to maintain orthogonality between transmit antenna ports at every downlink slot. If this scheme is applied to the massive MIMO system without modification, the number of CSI-RSs required is equal to the number of transmit antennas, and thus, a few hundred REs are allocated for CSI-RS transmission. This causes a reduction in the number of REs for data transmission, resulting in a reduction in downlink transmission capacity. Furthermore, the CSI-RS transmission at low power level per transmit antenna significantly decreases the CSI-RS reception performance.